Change default email client

Is it possible to change the default email client in EF?

I don’t use Mail, I would like to open the message in Gyazmail when I double click a mail in EF.

Sorry, it’s not. EagleFiler will use your preferred mail client when you use the Reply to Message command, but for opening messages (e.g. to access the attachments) Apple Mail is the only one that can do what EagleFiler needs. Long-term, I plan to improve EagleFiler’s built-in message viewer so that there’s no need to open messages in a mail client.